1. Field of the Invention
The present invention relates to sound signal processing, and, more particularly, to a method and an apparatus for estimating noise included in a sound signal.
2. Description of the Related Art
In sound signal processing for voice communication or for voice recognition that requires voice enhancement, it is important to estimate and remove noise included in a voice signal. Accordingly, schemes for estimating noise have been being proposed and used. For example, to estimate noise, one scheme first estimates the noise during a definite time interval, i.e. a period, in which a voice does not exist before the voice is input, and once the voice is input, a signal to reduce the estimated noise is applied. In another scheme, a voice is distinguished from a non-voice by using Voice Activity Detection (VAD), and then noise is estimated during a non-voice period. There is also a minimum statistics-based noise estimation scheme in which, based on characteristics of a voice spectral energy in a voice period being larger than spectral energy of noise and of a pronunciation period of a voice word corresponds to 0.7 to 1.3 seconds, values representing minimum energy in a given period are estimated to be noise. In a still further scheme, an approximate determination is made of the probability regarding whether a voice exists, to estimate noise during a period in which Voice Presence Probability (VPP) is large, whereas noise is not estimated during a period in which the VPP is small.
However, the above conventional noise estimation schemes have drawbacks in that they cannot detect changes of non-stationary noise, to reflect the changes in noise estimation. For example, inaccurate noise such as ambient audio sound that is abruptly generated in real life, or noise including a sound generated when a door is closed, a sound of footsteps, etc., having a short time duration but as also having a similarly large magnitude of energy as that of voice energy, cannot be effectively estimated. Hence, problems arise in that inaccurate noise estimation causes a problem of residual noise. Residual noise causes inconvenience of hearing to a user in voice communication or malfunction of a voice recognizing device, which degrades the performance of a voice recognizing product.
The reason conventional noise estimation schemes have the above problems is that when a scheme of processing a subsequent voice signal with reference to a result in a voice period previously processed, noise that is not the same as previous noise in a relevant period may exist, and when a scheme of estimating noise during only a relevant period with approximate prediction of a period in which noise exists, there is a limit for accurately estimating a period in which noise exists. Also, since a scheme for distinguishing between a voice and a non-voice by using a difference between the magnitudes of energy of respective signals or Signal-to-Noise Ratio (SNR), i.e. when a scheme for recognizing a period as a voice period if the value such as a difference between the magnitudes of energy of respective signals or Signal-to-Noise Ratio (SNR) is large and for regarding a period as a non-voice period if the value is small, if ambient noise having energy whose magnitude is similar to that of energy of a voice is input, noise estimation is not implemented, and, accordingly, a noise spectrum is not updated.